Public awareness of the petition site is growing much, much more rapidly than the number of eligible voters in the U.S. Thus it makes more sense to tie the required number of signatures to the number of people who know about the petitions site, which is essentially what they are doing.
For example, they might have internal targets of a fixed number of successful petitions per month, or a fixed fraction of petitions being successful. This makes sense. To maintain these targets the number of required signatures would be periodically adjusted.
